The proposed research is concerned with brain activity and behavior in the mentally retarded. The technique used to assess brain function is the sensory evoked response. Emphasis is placed upon the use of a wide variety of stimuli in three different sensory modalities: visual, auditory and somathetic. Although certain issues specific to hypothesized defects of sensory neural processing in Down's syndrome will be evaluated, data will be obtained from a wide variety of diagnostic groups. Evoked response and performance will be compared in at least the following six experimental paradigms. The methods and objectives of these are: 1. To expand the existing cross-sectional data base of resting EEG records obtained during eyes open and eyes closed conditions (with and without a simple motor task). This continuing data collection will build upon efforts begun in 1972. 2. Compare patterns of somatosensory evoked response with tests of tactile discrimination in order to gain a better understanding of why some subjects make many more errors in tests of tactile discrimination. 3. Assess the effects of both temporal and complexity dimensions of sensory stimulation upon neural activity in the mentally retarded using paired stimulus recovery functions. 4. Assess the characteristic properties of orientation and spatial frequency "channels." Such research may give clues as to the bandwidth of visual sensory information processing in the mentally retarded. 5. Assess the responsiveness of the visual system to chromatic and patterned light stimulation and to compare these data with the strength of a perceptual aftereffect (McCollough effect). 6. To assess lateralized cerebral processes relating to speech and language so as to gain insights into "higher" mental functioning in the mentally retarded.